The solar spots vary in a fairly regular period of eleven to twelve years. In certain years, _e.g._, 1893, they are vast, numerous and frequent; in other years, _e.g._, 1901, they are few and insignificant. The statistics are very carefully preserved. Here, for instance, is the surface showing sun-spots expressed in millionths of the extent of the visible solar surface: 1889 78 1890 99 1891 569 1892 1,214 1893 1,464 1895 974 1896 543 1897 514 1898 375 1899 111 1900 75 1901 29 1902 62 The years 1889 and 1901 were _minima_; the year 1893 a _maximum_. It is a curious fact that terrestrial magnetism and the boreal auroras exhibit an oscillation parallel to that of the solar spots, and apparently the same occurs with regard to temperature. We must regard our sun as a globe of gas in a state of combustion, burning at high temperature, and giving off a prodigious amount of heat and light. The dazzling surface of this globe is called a _photosphere_ (light sphere). It is in perpetual motion, like the waves of an ocean of fire, whose roseate and transparent flames measure some 15,000 kilometers (9,300 miles) in height. This stratum of rose-colored flames has received the name of _chromosphere_ (color sphere). It is transparent; it is not directly visible, but is seen only during the total eclipses of the Sun, when the dazzling disk of that luminary is entirely concealed by the Moon; or with the aid of the spectroscope. The part of the Sun that we see is its luminous surface, or photosphere. From this agitated surface there is a constant ejection of gigantic eruptions, immense jets of flame, geysers of fire, projected at a terrific speed to prodigious heights. For years astronomers were greatly perplexed as to the nature of these incandescent masses, known as prominences, which shot out like fireworks, and were only visible during the total eclipses of the Sun. But now, thanks to an ingenious invention of Janssen and Lockyer, these eruptions can be observed every day in the spectroscope, and have been registered since 1868, more particularly in Rome and in Catania, where the Society of Spectroscopists was founded with this especial object, and publishes monthly bulletins in statistics of the health of the Sun. These prominences assume all imaginable forms, and often resemble our own storm-clouds; they rise above the chromosphere with incredible velocity, often exceeding 200 kilometers (124 miles) per second, and are carried up to the amazing height of 300,000 kilometers (186,000 miles). [Illustration: FIG. 31.--Rose-colored solar flames 228,000 kilometers (141,500 miles) in height, _i.e._, 18 times the diameter of the Earth.]